The only way to lose body mass is to eat fewer calories than your body needs to consume for its size and activity level (water isn't body mass, all these jokers who get on a diet of like 50g of carbohydrates per day and then clap their flippers together when the scale is 10 pounds lighter in a few days don't understand what's going on). There's no way, no matter what you eat or in what proportion, to avoid losing some amount of muscle, and some amount of fat, when losing actual body mass. You can influence this somewhat, 70:30, maybe 80:20 if you're lucky, by eating enough protein, which most people don't. The Atkins diet, keto diets, etc., they work insofar as they trick people into eating more protein than they otherwise would, and fewer carbs than they otherwise would, because the typical person would probably like 60g of protein and 500g of carb per day, when it should really be closer to a 1:1 ratio between the 2. But the people out there eating like >200g of protein per day are basically just wasting their money unless they're a very large and elite athlete (proteins are expensive, after all). No one can show any marginal benefit at all for more than about 1.8g/kg (0.8g/lb) protein to body mass ratio, unless you're on steroids. The average guy, about 5'10", if he's 200 pounds, 10% body fat, most would agree he looks pretty athletic, and even that person only needs to eat about 160g of protein per day, but he also needs to eat about that many carbs per day, or he'd get his ass kicked at just about any athletic competition you can think of. So there's the rub, is 160g of carbs per day "low?" Some poor misguided souls try to eat 1/3 of that long-term (short-term good to manipulate water weight, which really only matters to sports with a weigh-in), so to them it seems high. But 160's way lower than non/weakly-dieting people tend to eat left to their own devices.